Minimizing pressure surges in a fluidized hydrocarbon conversion process



Jan 31 1961 J. w. JEWELL 2 MINIMIZING PRESSURE SURGES 1N AFLUIDIZED970,103

HYDROCARBON CONVERSION PROCESS Orlginal Filed July 1. 1953 VENT ToATMOSPHERE 36 l35 PRODUCT yRECOVERY SYSTEM REACTOR I3 olL FEED IN1/EMDR.JOSEPH W. JEWELL STR IPPING GAS "7.

INERT GAS REGEN- AERAToR FLuE COOLING` TUBES AIR STEAM INERT GAS OILFEED A TORN EYS MINIMIZING PRESSURE SURGES LN. A FLUIDZED `HYDRCARBONCONVERSN PROCESS Joseph W. Jewell, Summit, NJ., assigner to The M. W.Kellogg Company, Jersey City, NJ., a corporation of Delaware Originalapplication July 1, 1953, Ser. No. 365,435, new Patent No. 2,891,907,`dated June 23, 1959. Divided and this application Sept. 22, 1958, Ser.No. 762,370

Claims. (Cl. 20S- 164) This invention relates to an improved iluidsystem, and more particularly, it pertains to method and means forimprovements in a fluid operation wherein catalyst flows from oneprocessing zone to another whereby the adverse effects of pressuresurges are significantly minimized.

This application is a division of my prior and copending applicationSerial No. 365,435, tiled July 1, 1953, now Patent No. 2,891,907.

In certain designs of fluid reaction systems, catalyst is circulatedfrom one processing zone to another as a dense fluidized mass at arelatively low rate, consequently, the regeneration and transfer line,particularly the latter, does not contain a suthcient amount of catalystto absorb any appreciable quantity of heat. The process may operate at asuperatmospheric pressure, for example, about 250 p.s.i.g., and with areversal of pressure, a backow of gaseous material can take place. Thebacktlow can result in overheating the transfer line to the point offailure by the commingling of oxygen containing gas from the regeneratorwith hydrocarbon and/or hydrogen flowing with the catalyst from theother processing vessel. The effect is only relatively less seriousshould the reactant materials backflow from the reactor through theregenerated catalyst transfer line and into the regenerator as resultantlocal overheating may damage the catalyst. In the transfer lines fromthe stripper to the regenerator to the reactor, the masses of catalystare flowing downwardly. Since the catalyst flows as a dense fluidizedmass in the transfer lines, there is a strong possibility of catalystbridging due to the compressive effect of a pressure surge on thealready dense phase in the lines. When `catalyst bridges in a transferline, the required pressure differential across the control valve in theline is lost, hence, gaseous material bacldlows from the processingvessel to which the catalyst normally feeds. In the case of regenerationgas bacleowing into the spent catalyst transfer line, the oxygen in thegas will react with the hydrogen and/or hydrocarbons present in the lineand cause excessive temperature rises by reason that there is little orno effective means of diluting or dissipating the heat generated. Thiscan readily cause the transfer line to melt, and further, the lire oncestarted may spread rapidly to the other processing vessels. It isapparent, therefore, that in designs or processes where the vessels arearranged to provide downflowing dense uidized masses of catalyst, thereis a need for an operation which eliminates or substantially overcomesthe disadvantages inherent in such schemes. A

An object of this invention is to provide improvements in fluid systemsin which catalyst ows from one processing vessel to another,particularly in cases where the quantity of catalyst circulating through`.the regeneration system is relatively small.

Another object .of this invention is to provide improved method andmeans for a fluid system where catalyst must be transferred betweenprocessing vessels `and the danger of 'backflow is significantlyminimized.

2,970,103 Patented dan. 3l, wel

,time

Still another object of this invention is to provide improved method andmeans for fluid hydroforming of naphtha fractions.

Other objects and advantages of this invention will become apparent fromthe following description and explanation thereof.

in accordance with the present invention, the danger of backflow whichis hazard in a downowing dense aerated mass of finely divided contactmaterial passing from one processing zone to another in a fluid systemis significantly minimized by passing or injecting an inert or aerationgas into said downflowing mass of material in a quantity suiceint toreduce the flowing density such that the danger from backtlow issignificantly minimized. The quantity of gas introduced for the purposeof this invention is substantially more than is used for aeration assuch, because in the latter case, the object is to add only enough gasso as to prevent density increases in the flowing mass of contactmaterial.

In the practice of this invention, the inert gas supply is introduced ata pressure which is greater than the pressure existing in the transferzone in order that the gas can also serve to buffer the effect of a backsurge. lf a back surge occurs in the transfer zone, the incoming gaswill increase the pressure in the transfer zone near the injection pointof the inert gas, and so offset the effect of the back surge. Thiseffect is produced by maintaining the rate of aeration gas constant. Themechanism by which the aeration gas serves to offset the `effect of backsurging suggests injecting this inert gas at a point in the downflowingdense mass which iS uppermost relative to the discharge end of thetransfer zone. For example, in a transfer line connecting an elevatedstripping zone with a lower positioned regeneration zone, a slide valvewould be installed for the purpose of flow control as close to thebeginning of the transfer line or as near to the stripping zone aspossible, and the carrying or inert gas would be injected as close tothe downstream side of the slide valve as is practical. Since thequantity of gas injected into the transfer zone is relatively large ascompared to simple aeration, in the event of a backliow of undesiredgaseous material into the transfer zone, this aeration or -inert gasserves also as a diluting medium to minimize the effect of burningand/or explosion. In the event of combustion by reason of backflow, theinert gas serves as a heat dilnent in suppressing undue temperaturerises. Further, this inert gas serves to create a buffer or atmosphereof inert gas against which the backllowing gaseous material would haveto flow to reach the higher processing zone, be.- cause the backowmaterial must be motivated by a force or pressure which is greater thanthe pressure under which the inert gas exists at the supply source.

In the determination of the quantity of inert or aeration gas to beused, the criterion yto follow is to seek the amount of gas which willdilute the solid material to a density such as .to overcome the tendencyof the material to bridge when the pressure fluctuates by about l to 20p.s.i.g., more usually, about 2 to 5 p.s.i.g, in each or both of theprocessing zones, which are interconnected by means of the particulartransfer zone. Usually, it is found that the quantity of inert gas issufficient to reduce the density of the downflowing, dense fluidizedmass to about 20 to 50%, preferably about 20 to 35%, of the value atwhich it exists just prior to dilution with inert gas. The inert oraeration gas should be one which does not `react with any of thematerials being used in the processing zones. In the ease of hydrocarbonconversion processes, steam -serves very well for this purpose. Othergases which can be used are, for example, nitrogen, carbon dioxide, fluegas from the combustion of carbonaceous material inthe same process,etc.

The technique of this invention can be applied to any process in whichgaseous materials are contacted with finely divided solid material inseparate processing Zones; and the gaseous materials in the respectiveprocessing zones should remain separated from each other for any reason.The contact material is passed from one processing zone to yanother as adownflowing dense fluidized mass, thus providing the type of situationfor which the present invention has particular applicability. Ingeneral, the contact material has a particle size of about to 1000microns, more usually, about 0 to about 100 microns, and this materialcan be a catalyst, adsorptive or absorptive solids, heating medium,cooling medium, etc. The processes for which this invention has specialutility are the conversion of hydrocarbons such as, for example,catalytic cracking, hydroforming, cracking under hydrogen pressure,desulfurization, hydrogenation, dehydrogenation, etc. As previouslymentioned, certain designs for fluid hydroforming are particularlysuited for adaptation to this invention.

Further description of this invention will be made with reference tofluid hydroforming, however, this is done for the purpose ofillustration, and so no undue limitations or restrictions are to beimposed by reason thereof on the scope of this invention.

This invention can be applied as an improvement to a fluid hydroforrningsystem in which the processing Vvessels are arranged to provide for thetransfer of catalyst from one zone to another as a downtiowing denseiiuidized mass. An example of a iiuid hydroforming system for which thepresent invention can be used as an improvement involves transportingspent catalyst from the bottom of the reaction zone as a suspension to asuperimposed stripping zone. After stripping, the catalyst is passed asa downflowing dense uidized mass to a regeneration zone, and thence, theregenerated catalyst is passed as a downflowing dense iiuidized mass tothe lower portion of the reactor catalyst bed. In any case, it is notedthat the transfer zones contain downflowing dense fluidized masses ofcatalyst. In these iiuid systems, the angle of flow of solid materialcan vary over the range of being just greater than 45 relative to ahorizontal plane or datum, and it can be vertical or 90 relative to thesame base level. More usually, such transfer lines or zones are designedto be inclined at least about 60"V from a horizontal plane, andpreferably at least about 75 from .the same plane in order to avoid thefinely'divided material settling in the transfer zone.

The increased angle of iiow from the horizontal can aid .materially inmaintaining a uniform liowing condition for the finely divided material,and this fact is exploited in the commercial design of transfer lines.By means of this invention, lower angles of ow, e.g., an angle justgreater than zero or at least 30 from a horizontal level, can be used.At angles of less than 45 for this invention, it is preferred to uselinear flow rates of about 10 to 25 feet per second.

T-he downiiowing mass of dense aerated finely divided material can bediluted by means of the present invention to provide a lean phase ofmaterial in the transfer line. The point of demarcation between denseand lean phases is not easily defined, however, for the purpose of thisinvention, these phases can be defined in terms of the percentage ofbulk or settled density they represent. Generally, the desirable leanphase is not more than about 5 to about 35% of the settled density, andthe dense phase has a density above this figure. There is a range ofdensities which might be considered as intermediate between dense orlean phase, however, for the 4 aspect of this invention, the density ofthe downowing mass can be small enough to create no uistatic pressure inthe sense of a standpipe for .the purpose of this invention. f

In a hydroforming process, the catalyst is one which has the propertiesof forming aromatics or it is an aromatizing catalyst which is capableof dehydrogenating naphthenes, isomerizing and cracking acyclic andcyclic hydrocarbons, and dehydrocyclizing acyclic hydrocarbons.lCatalysts found useful for this purpose include the suliides and/0roxides of the left hand elements of group VI, the oxides and/or sulidesof the group V metals, the noble metals of group VIII, etc. Specificexamples of catalytic elements having the properties mentioned above aremolybdenum trioxide, tungsten oxide, chromia, platinum, palladium,vanadium oxide, etc. These catalytic elements are used alone or they aresupported on a suitable carrier material such as, for example, alumina,zinc spinel, silica, magnesia, titania, zirconia, silica-alumina,bauxite, charcoal, alumina-thoria, etc. Another class of catalyticelements which are specially effective for the hydroforming reaction arethe heteropoly acids such as, for example, phosphomolybdic acid,silicomolybdic acid, germanomolybdic acid, chromiomolybdic acid, etc.These catalytic elements may be used alone or they can be supported onthe carrier materials mentioned above. In general, the catalytic elementconstitutes about 0.1 to about 50% by weight, more usually, about 0.5 toabout 20% by weight based on the total catalyst.

The feed stock to be reformed by means of the present invention is alight hydrocarbon oil and includes, for

example, gasoline, naphtha and kerosene. The light hydrocarbon oil canhave an initial boiling point of about 85 to about 325 F. and an endpoint of about 300 to about 500 F. In the case of reforming a naphthafraction, it is preferred to employ a naphtha having an initial boilingpoint of about 100 to about 250 F. and an end point of about 350 toabout 450 F. Generally, the light hydrocarbon oil to be reformed has aWatson characterization factor of about 11.50 to about 12.20. The feedmaterial can be one which is a straight run or virgin stock, a crackedstock which is derived from a thermal or catalytic cracking operation,or a mixture or a blend of straight run and cracked stocks. Accordingly,.the octane number ofthe feed material can be at least 5 CFRR clear, ormore usually, about 20 to about 70 CFRR clear and the olefin content ofthe oil can vary from about 0 to about 30 mol percent. This lighthydrocarbon oil can be derived from any type of crude oil, consequently,it can contain sulfur in the amount of 0 to about 3.0% by weight.

The light hydrocarbon oil is reformed under conditions which can involvethe net consumption or net producpurpose of this invention, thesedensities will be included tion of hydrogen. A system involving the netproduction of hydrogen is referred to hereunder as hydroforming, and itis operated under such conditions that the quantity of hydrogen producedis suiiicient to sustain the process without the need for extraneoushydrogen. Generally, for the reforming of light hydrocarbon oils, atemperature of about 750 to about 1100o F. is employed. At thistemperature, the pressure of the operation is generally maintained atabout 25 to about 1000 p.s.i.g. The quantity of oil processed relativeto the amount of catalyst employed is measured in terms of weight spacevelocity, that is, the pounds of oil feed on an hourly basis charged tothe reaction zone per pound of catalyst which is present therein. Theweight space velocity can vary from about 0.05 to about l0. Theconcentration of hydrogen to be maintained in the reactor feed isusually measured in terms of the standard cubic feet of hydrogen(measured at 60 F. and 760 mm.) per barrel vof oil feed charged (lbarrel=42 gallons). On this basis, the hydrogen rate is about 500 toabout 20,000 s.c.f.b. Another method of indicating the quantity ofhydrogen which `of catalyst fines.

nemica can be present during the reforming operation is by means ofhydrogen partial pressure. In this regard, the hydrogen partial pressureis about l5 to about 950 p.s.i.a. in the reaction zone, based on inletconditions.

In a hydroforming operation, the reaction conditions fall within theranges specified hereinabove, however, they are selected on the basis ofobtaining a net production of hydrogen. A preferred hydroforming processinvolves a temperature of about 850 to about l050 F.; a pressure ofabout 50 to about 500 p.s.i.g.; a weight space velocity of about 0.1 toabout 3; a hydrogen rate in the reactor feed of about 1000 to about 7500s.c.f.b. and a hydrogen partial pressure of at least about 2S p.s.i.a.,and up to the point at which hydrogen is consumed.

The regeneration treatment for the removal of carbonaceous material fromthe contact material is conducted at a temperature of about 800 to aboutl200 F. and preferably, at a temperature of about 950 to about l150 F.This treatment is effected with an oxygen containing gas, e.g., oxygen,air, diluted air having about l to about by volume of oxygen, etc. Theseconditions can be used for either partial or complete regeneration and,as previously mentioned, the temperature can be higher or lower for onetreatment over the other, depending upon the reaction desired.

In the accompanying drawing, improved method and means for the operationof fluid hydroforming are illustrated. For a better understanding of thepresent invention, these improvements will be discussed below.

In the drawing, reactor 5 is a vertical, cylindrical vessel having agrid plate 7 in lthe bottom -part thereof upon which there is situated alluidized mass of finely divided catalyst comprised of 9% M003 supportedon alumina shown as having a level 9. Split streamsof naphtha vapor areintroduced by means of lines 11 and 13 and conical distributors 15 andi7, respectively, into the bottom part of the reactor, such that eachstream is discharged into one-half of the cross-sectional area of thereaction Zone. The total naphtha feed is introduced at the rate of 2000barrels per stream day and at a temperature of about 930 F. and byvirtue of the quantity of catalyst which Vis present in the reactionzone, there is provided a weight space velocity of about 0.45. Thetemperature `in the reaction zone is maintained at about 930 F. under atotal pressure of about 250 p.s.i.g. Recycle gas containingapproximately 60% by volume of hydrogen is fed into the bottom end ofthe reactor Vby means of line 19 at the rate of 5000 s.c.f.b. Therecycle gas is fed below the grid plate 7 in the reactor in order thatthe grid plateI may serve to distribute the gas uniformly over thecrosssectional area of the reaction zone. This recycle gas is introducedat a temperature of about l250 F. and it serves to supply a substantialamount of the endothermic heat of reaction which is required for thehydroforming process. The reactionproduct becomes disengaged from thecatalyst bed, and yit usually Acarries a small amount The productmaterial leaves Ythe -reac tor by irst passing through a cycloneseparator 2l which is in series with a second cyclone 23. The separatedcatalyst lines are returned to the reaction zone by first passingthrough diplegs Z5 and 27 of `cyclones 21 and 23, respectively, whichdiplegs are in turn connected `to vertical, downlow conduit 29 throughwhich excess catalyst and the gas used to lift the catalyst in riser 42llow from the stripper hopper downwardly to the reactor bed. Thereaction product substantially free of catalyst nes rst passes throughvertical line 31 of cyclone 23 and then it is discharged from the systemby means of line 33. In ,the reactor product discharge line 33, there isinstalled a control valve 34 for the purpose of Vmaintaining Vthedesired `reaction pressure in the reactor 5. It should be understoodthat from the standpoint of optimum pressure control in the system, itis preferred to have a low pressure `drop control valve, such asvalve34, positioned in discharge line 33 ahead of any other equipmentand close `to the reactor. The dissipation of pressure from essentiallyoperating level to a substantially lower level, such as atmosphericpressure, may be effected at a remote point from the position of the lowpressure drop control valve, such that the latter control valve servesto dampen any variations in pressure in the system which may tend toadversely influence .the reactor pressure, hence, the result is that thereactor undergoes less pressure fluctuation than is experienced normallywithout the low pressure drop control valve. This additional provisionfor close control permits the use of lower pressure drops acrosscatalyst transfer line control valves with less danger of backilow. Inthe drawing, the arrangement is illustrated by means of a productrecovery system shown schematicallyas 35, and downstream of the productrecovery system, the net production of normally gaseous product from therecovery system llows through line 36 prior lto being vented to theatmosphere through high pressure drop control valve 37. By controllingthe pressure of the reactor in this fashion, there is less of a pressurefluctuation therein by reason that valve 34 will act quickly to restorethe desired pressure level without the time lag normally associated witha control system in which a single control valve is employed fiordissipating operating pressure to the atmosphere. Therefore, in systemsusing a single control means, it operates at a much higher pressure dropand is more susceptible to appreciable pressure iluctuation. Therefore,by means of this installation, control valve 34 functions with a minimumof pressure drop in its operation, for example, about l to 5 p.s.i.g.This feature is highly important, because downilowing dense iluidizedmasses of catalysts in transfer zones are subjected to smaller pressurevariations and of less time duration by reason of the close control onfluctuations `of pressure in the reactor. Hence, there is less of atendency for the catalyst in the transfer lines to become deaerated dueto large pressure lluctuations in the processing vessels.

Spent catalyst is withdrawn from the bottom part of the reactor, belowthe entrance point of the naphtha feed and above the point of entranceof the recycle gas.v

The rate of withdrawal of spent catalyst is controlled by means of aplug valve 38 to which there is fed through the hollow stern anysuitable carrying gas, such as steam, by means of a supply line 40.Spent catalyst flowing through plug valve 3E is picked up by thecarrying gas, and it is transported upwardly as a suspension through averticalriser 42. There is superimposed on reactor 5' a catalyst hopperda, which is a vertical, cylindrical vessel. While this vessel 44 istermed a hopper, it should be understood that for the purpose of thisspecification and the appended claims, that this vessel does notnecessarily retain a bed of catalyst, but 4it serves primarily as aconduit for the downward flow of gases and solids. Hopper 44 isconnected to Vertical downcomer 29 by means of a reduced section 46. Avertical, cylindrical stripper 50 superimposes the `catalyst hopper 44.This stripper 50 contains a partition 52. which separates it from thehopper 44. In one side of the stripper there is situated a vertical,transverse baille 53 which forms a duct 55 by means of which excessspent catalyst lifted to the stripper hopper is returned to the catalysthopper 44. Generally, about l to about 40% by weight of the totalcatalyst passed upwardly in riser 42 is recycled to the reactor by meansof hopper 44. Preferably, about 5 to 15% by weight, on the same basis,is recycled. in an opposite position to duct SS a second transversebaille 57 of shorter length than transverse baille 53 is situated andthis forms a stripping well S9. Stripping gas is fed into the bottompart of stripping well 59 by means of line 61. A vertical conduit 63 isconnected .to partition 52 in the bottom of the stripper and it is inopen cornrnunication with hopper 44. The upper end of this conduit 63 issituated in the vapor space in the stripper above the catalyst bed level65. Any gaseous material is discharged therefrom through conduit 63and/or through duct 55. By the design of the stripper, a levelofcatalyst is always maintained therein by providing recycle v'of spentcatalyst through duct 55. Further, the transverse bai-lle 53 which formsduct 55 is of greater length than transverse baille 57 which formsstripping well 59.. In this manner, there is a positive andsubstantially uniform head of catalyst above the stripper well providinga reserve capacity of catalyst hold-up in a section of greatercross-sectional area, thus avoiding, to a great extent, the possibilityof losing the catalyst level in the stripper in case of normal changesin the flow of catalyst to the regenerator, which in turn would cause achange in the available pressure over the slide valve. Vertical riser42, which was previously mentioned, is the conduct by which spentcatalyst is transported as a suspension from the reactor and has itsupper end connected to partition 52 which forms the bottom of stripperhopper 50. Hence, spent catalyst is withdrawn from the reactor at agreater rate than is necessary for regeneration and restoration ofcatalytic activity. The recycle of spent catalyst also serves toincrease the internal circulation of catalyst particles within thereactor bed, thus resulting in a more uniform distribution of catalystparticles of different activity and a consequent improvement in reactorefficiency. The stripped catalyst Iis discharged from stripping well 59by means of a spent catalyst transfer line 67. Catalyst is dischargedfrom the stripping well, which is suicient to provide a catalyst to oilratio of about 0.4. On the other hand, the rate of spent catalystrecycled to the reaction zone constituted about 10% by weight of. thetotal catalyst passed upwardly in riser 42. In operation, spent catalystrecycle is combined with the stripped gaseous product material flowingdownwardly through duct 55, and thence, as a combined stream they flowdownwardly through hopper 44, downcorner 29 and then discharge justabove catalyst level 9 in the reactor. The pressure in the stripper ismaintained at 250 p.s.i.g. and at a temperature of 900 F. About 300pounds per hour of steam is fed into the stripping well 59 through line61.

The stripped spent catalyst is withdrawn from the stripping Well 59 as adense fluidized mass having a density of about 35 pounds per cubic foot.The rate of catalyst withdrawal is controlled by means of a slide valve'70 which is installed in the upper part of carrier line 67 in closeproximity to stripper well 59. Downstream of slide valve 70, anysuitable inert gas, such as steam, is fed into carrier line 67 by meansof line 71 at a rate sufiicient to reduce the ilowing density of thespent catalyst to about l pounds per cubic foot. The rate of inert gasbeing supplied to the downliowing spent catalyst mass is held constantat this iigure, and it is supplied from a source which is maintained ata pressure sufficiently higher than the system pressure to assureconstant ilow against normal iluctuation in the system pressure and toprovide a positive means of quickly blowing out any plug in the transferline. Carrier line 67 is joined with a regenerator 75. This carrier lineprojects a considerable distance within the regenerator proper above thebed level. The regenerator contains a dense fluid bed of catalyst havinga level 77. The heat of regeneration is removed by means of coolingtubes 79. Regeneration gas, air, is fed through a supply line 80 and aconical distributor 82 into the lower part of the catalyst bed in theregenerator. The air is introduced at a point which is about l0 feetabove the bottom of the regenerator vessel. At the bottom of theregenerator there is introduced steam by means of line 84 and conicaldistributor 86. The steam serves to strip from the regenerated catalystany oxygen which might be occluded therewith. That portion of thecatalyst bed which is situated between the steam inlet 84 and the airinlet 80 is a stripping zone for the regenerated catalyst. By strippingthe regenerated catalyst in this manner, there is provided a reserve ofcatalyst of lowV oxygen concentration such' that any pressureiiuctuation causing increased flow tothe reactor does` not introduceappreciable quantities of oxygen Vto the reactor. The temperature in there*- generator is maintained at 1100" F. and at a pressure in the top ofthe vessel of 250 p.s.i.g. lt is to be noted that the system beingdescribed has equal pressures in the regenerator, reactor and stripper.Such a scheme eliminates the need for a catalyst density in the catalysttransfer line which will act as a fluistatic head to produce a higherpressure at the outlet than at the inlet of such lines. And by thisarrangement the factors influencing the flow of catalyst in the transferlines from one side to the other of the system, are not in any wayaiected by varying bed levels in either the reactor or the regenerator.The ue gas is disengaged from the catalyst bed, and it contains a smallamount of entrained catalyst nes which is recovered to a substantialextent by means of cyclones and 92 which are connected in series. Theflue gas first passes through cyclone 90 and the gaseous materialdischarged from the first cyclone is passed to cyclone 92 by means ofline 94. The separated catalyst is returned from the cyclones 90 and 92by means of diplegs 96 and 98, respectively, to the lower end of carrierline 67 which depends from the top of the regenerator. The flue gas,substantially free of catalyst iines, is discharged from cyclone 92 bymeans of a line 100, and the gaseous material leaves the regeneratorsystem through a vent line 101.

There is installed in vent line 101, a control valve 102 which serves tomaintain the desired pressure in the'regenerator. As in the case of thereaction system, control valve 102 operates with a rather small pressurediiierential, hence, there is little time lag between the uctuaton ofthe pressure and the time when the control valve responds or functions.The advantage inV this type of control has been explained in connectionwith the reaction system. The flue gas then ows to a scrubber shownschematically as 103, from which it ows into a line 104. Line 104contains a control valve 105 which serves to dissipate pressure from anoperating level to atmospheric pressure. As previously indicated, thepressure existing at the top of the regenerator is maintained atapproximately 250 p.s.i.g. which is the same as the reactor pressure. Inthis manner, should there be a uctuation in catalyst circulation whichwould cause a backiiow of gaseous materials from one zero to the other,the backilow receives essentially no help from the pressure in either ofthe processing zones, because there is no pressure diierential betweenthem. The stripped regenerated catalyst is withdrawn through carrierline 110, which is connected to the bottom end of the regenerator 75,and in turn, it is conveyed to the reactor 5 at a point above the level9 of the catalyst bed situated therein. At the point where regeneratedcatalyst is discharged from the reactor, there is situated a baffle 112for the purpose, of preventing undue scattering of catalyst throughoutthe reactor above the catalyst level 9. Near the bottom of theregenerator, there is installed a valve 114 in carrier line for thepurpose of controlling automatically the rate of catalyst withdrawal. Ata short distance from the downstream side of valve 114, inert gas, suchas steam, is introduced through line 116. The regenerated catalyst iswithdrawn from the regenerator having a density of about 35 pounds percubic foot and by means of dilution of steam in carrier line 110, thisdensity is reduced to about 10 pounds per cubic foot.

From the description above, the improved system contains many advantagesover other existing designs for the same application. By means of thearrangement shown in the drawings, the spent catalyst is transported toan elevated position or stripper 50, such that a constant cat alystlevelis maintained therein by having bale 53 of greater height thanbafiie S7, and the relative heights of the equipment are such that therecan be continuous downow in lean phase in the transfer lines from thespent catalyst stripper to the regenerator and back to the reactor, withno influence of the bed levels on the characteristics of flow in theselines, and with no appreciable influence of conditions in the transferlines on the control pressure drop across the slide valves. The pressurebalance of the system permits the installation of control valves in thetop of the transfer lines rather than the bottom part of the lines, thusthe system can operate without standpipes. The dilution of downowingcatalyst in the transfer lines provides flowing velocities of about 3 to20 feet per second, preferably, about 5 to 10 feet per second,consequently, there is less danger of plugging and the velocities can below enough to substantially avoid an erosion problem. The stripping zonein the bottom of the regenerator provides a mass of catalyst which canabsorb heat in the case of combustion from backow of reactant materialsfrom the reactor. This is also true of the stripper by reason of thecatalyst hold-up therein.

Having thus provided a description of my invention, it should beunderstood that no undue limitations or restrictions are to be imposedby reason thereof, but that the scope of this invention is defined bythe appended claims.

I claim:

1. A process which comprises contacting a hydrocarbon reactant in thepresence of hydrogen with a mass of finely divided contact material in areaction zone to produce a vaporous reaction product, withdrawing aportion of said reaction product from said reaction zone, passing theproduct thus withdrawn through a first transfer zone into a productrecovery system, variably constricting said first transfer zone at apoint between said reaction zone and said product recovery system toimpose a pressure drop upon the system, withdrawing products from saidrecovery system through a second transfer zone, and variablyconstricting said second transfer zone at a` point between said productrecovery system and the point of product withdrawal from said secondtransfer zone to impose a pressure drop upon the system greater thanthat imposed by the constriction of said first transfer zone and wherebythe adverse effect of pressure surges is significantly minimized.

2. A process which comprises contacting a hydrocarbon reactant in thepresence of hydrogen with a dense bed of `finely divided solid contactmaterial in a reaction zone to produce a vaporous reaction product,withdrawing a portion of said reaction product from said reaction zone,passing the product thus withdrawn through a first transfer zone into aproduct recovery system, variably constricting said first transfer zoneat a point between said reaction zone and said product recovery systemto impose a pressure drop upon the system, withdrawing products fromsaid recovery system through a second transfer zone, and variablyconstricting said second transfer zone at a point between said productrecovery system and the point of product withdrawal from said secondtransfer zone to impose a pressure drop upon the system greater thanthat imposed by the constriction of said first transfer zone and wherebythe adverse effect of pressure surges is significantly minimized.

3. A process which comprises contacting a hydrocarbon reactant in thepresence of hydrogen with a dense bed of finely divided solid contactmaterial in a reaction zone at superatmospheric pressure to produce avaporous reaction product, withdrawing a portion of said reactionproduct from said reaction zone, pass-ing the product thus withdrawnthrough a first transfer zone into a product recovery system, variablyconstricting said first transfer zone at a point between said reactionzone and said product recovery system to impose a pressure drop upon thesystem, withdrawing products from said recovery system through a secondtransfer zone, and variably constricting said second transfer zone at apoint between said product recovery system and the point of productwithdrawal from said second transfer zone to impose a pressure drop uponthe system greater than that imposed by the constriction of said firsttransfer zone and whereby the adverse effect of pressure surges issignificantly minimized.

4. A hydrocarbon conversion process which comprises contacting ahydrocarbon reactant in the presence of hydrogen with a dense bed offinely divided solid contact material in a reaction zone to produce avaporous reaction product, withdrawing a portion of said reactionproduct from said reaction zone, passing the product thus withdrawnthrough a first transfer zone into a product recovery system, variablyconstricting said first transfer zone at a point between said reactionzone and said product recovery system to impose a pressure drop upon thesystem, withdrawing products from said recovery system through a secondtransfer zone, and variably constricting said second transfer zone at apoint between said product recovery system and the point of productwithdrawal from said second transfer zone to impose a pressure drop uponthe system greater than that imposed by the constriction of said firsttransfer zone and whereby the adverse effect of pressure surges issignificantly minimized.

5. A hydrocarbon conversion process which comprises contacting ahydrocarbon reactant in the presence of hydrogen with a `dense bed offinely divided solid contact material in a reaction zone atsuperatmospheric pressure to produce a vaporous reaction product,lwithdrawing a portion of said product from said reaction zone, passingthe product thus withdrawn through a first transfer zone into a productrecovery system, variably constricting said first transfer zone at apoint between said reaction zone and said product recovery system toimpose a pressure drop upon the system, withdrawing products from saidrecovery system through a second transfer zone, and variablyconstricting said second transfer zone at a point between said productrecovery system and the point of product withdrawal from said secondtransfer Vzone to impose a pressure drop upon the system greater thanthat imposed by the constriction of said first transfer zone and wherebythe effect of pressure surges is significantly minimized.

References Cited in the file of this patent UNITED STATES PATENTS2,460,356 Liedhohn July 10, 1951 2,598,058 Hunter May 27, 1952 2,689,823Hardy et al. Sept. 2l, 1954 2,797,189 Virgil June 25, 1957

5. A HYDROCARBON CONVERSION PROCESS WHICH COMPRISES CONTACTING AHYDROCARBON REACTANT IN THE PRESENCE OF HYDROGEN WITH A DENSE BED OFFINELY DIVIDED SOLID CONTACT MATERIAL IN A REACTION ZONE ATSUPERATMOSPHERIC PRESSURE TO PRODUCE A VAPOROUS REACTION PRODUCT,WITHDRAWING A PORTION OF SAID PRODUCT FROM SAID REACTION ZONE, PASSINGTHE PRODUCT THUS WITHDRAWN THROUGH A FIRST TRANSFER ZONE INTO A PRODUCTRECOVERY SYSTEM, VARIABLY CONSTRICTING SAID FIRST TRANSFER ZONE AT APOINT BETWEEN SAID REACTION ZONE AND SAID PRODUCT RECOVERY SYSTEM TOIMPOSE A PRESSURE DROP UPON THE SYSTEM, WITHDRAWING PRODUCTS FROM SAIDRECOVERY SYSTEM THROUGH A SECOND TRANSFER ZONE, AND VARIABLYCONSTRICTING SAID SECOND TRANSFER ZONE AT A POINT BETWEEN SAID PRODUCTRECOVERY SYSTEM AND THE POINT OF PRODUCT WITHDRAWAL FROM SAID SECONDTRANSFER ZONE TO IMPOSE A PRESSURE DROP UPON THE SYSTEM GREATER THANTHAT IMPOSED BY THE CONSTRICTION OF SAID FIRST TRANSFER